The present invention relates to an X-ray detector which minimizes the effects of leakage X-rays in ionization chamber detector systems where collimation is used to define resolution.
Industrial X-ray Computerized Tomography (CT) systems are designed to provide images of the internal structure of manufactured parts. In order to effectively image small structures or defects, high resolution measurements are required. Industrial inspection applications generally require higher resolution and higher energy X-rays compared to medical applications. Inherent in a high resolution measurement is a small aperture, and resultant limited number of X-rays passing along the desired inspection path.
In systems where collimation is used to define resolution, ordinarily the collection volume of each detector element is significantly larger than the primary beam volume at the collector. Typically, the collection volume is from fifteen to fifty times the primary beam volume. As a practical matter, collimators do not serve their function perfectly. Thus, a typical collimator attenuation factor is fifty, and radiation, which becomes leakage radiation, passes through portions of the collimator other than the desired beam-defining apertures.
Leakage radiation, which may be viewed as "noise", is accordingly a serious problem. Although the leakage radiation is greatly attenuated, the total collection volume sensitive to leakage radiation results in a relatively poor signal-to-noise ratio. Leakage radiation must be limited to very small values in order to achieve adequate signal-to-noise ratios for effective measurements.
More particularly, a collimator/detector system of the general type with which the present invention is concerned is disclosed in commonly assigned application Ser. No. 875,021, filed , June 16, 1986, now U.S. Pat. No. 4,638,499 which is a continuation of Ser. No. 638,171, filed Aug. 6, 1984, now abandoned. As described in that application, a typical X-ray detector array for use in industrial CT systems is of the type described in Houston et al., U.S. Pat. No. 4,394,578, and includes, within an ionization chamber containing pressurized xenon gas, a high voltage plate at a fixed separation from a printed circuit detector board carrying a linear array of individual collector electrodes held generally at ground potential and connected to detector electronics. Incident X-rays enter between the high voltage plate and the detector board in a beam along an axis between and parallel to the voltage plate and detector board and ionized xenon atoms, producing positive and negative xenon. Assuming the voltage plate carries a negative charge, the positive ions are attracted to it. The negative ions are attracted to the collector electrodes, and transferred to a charge-to-voltage converter, which may be viewed as the first stage of a data acquisition system. The resulting voltage is digitized and transferred to a computer. A typical detector board contains a plurality of metallized collector electrode regions, separated by narrow insulating strips.
The resolution of the detector alone is determined, to first order, by the width of the collector electrodes. As described in application Ser. No. 875,021, now U.S. Pat. No. 4,638,499, such a detector, without more, does not provide satisfactory resolution. In order to achieve resolutions on the order of 10 mils and less, the beam width of the incident X-ray beam is appropriately limited by means of the collimator.
The present invention further improves such systems to minimize the effects of leakage radiation on the measured signal-to-noise ratio. The invention may be viewed as aiding the function of the collimator, making it more effective. The present invention also serves to reduce crosstalk between adjacent detector channels.
Although disclosed herein in the context of a limited angle industrial X-ray CT inspection system for providing high resolution images of long, thin metal objects, the invention is useful in any situation where collimation is used to define resolution, that is, where collection volume is larger than primary beam volume at the collector. Application in specialized medical X-ray systems is also possible.